Early stages of innervation and sensory cell differentiation in the human fetal organ of Corti.

Early development of the human organ of Corti was investigated at the light and electron microscopic level. In 9-week fetal material the cochlea was completely coiled and nerve fibers penetrated into an undifferentiated Corti's primordium. By week 10 a single cell in radial sections, presumed to be the inner hair cell (IHC), could be found with many nerve fibers surrounding and contacting its base. It is possible to identify outer hair cells by the end of the 11th week while IHC development continues. During the 12th week stereocilia appear on IHCs and synaptic specializations could be found at the junctions between both types of hair cell and afferent dendrites. The first appearance of the efferent fibers beneath IHCs were also observed during week twelve.